The invention relates to cathodic electrodeposition (CED) coating agents containing bismuth complexes, the preparation and use thereof in processes for coating electrically conductive substrates by CED coating.
The use of various bismuth salts as a constituent in lead-free CED coating agents is described repeatedly in the patent literature, for example, in U.S. Pat. Nos. 5,936,013, 5,702,581, 5,554,700, 5,908,912, 6,174,422, 5,670,441, WO 96/10057, U.S. Pat. No. 5,972,189, WO 00/50522, U.S. Pat. No. 6,265,079, EP 1 041 125, WO 00/47642, WO 01/51570. DE-A 44 34 593 and U.S. Pat. No. 5,702,581 also mention the possibility of using bismuth complexes, for example, bismuth-acetylacetone chelates as a constituent in CED coating agents, but emphasize at the same time that the use of bismuth salts of organic monocarboxylic acids is preferred and the use of bismuth salts of hydroxycarboxylic acids is particularly preferred.
As a rule, metal substrates to be coated with CED coating agents with a view to obtaining a conversion layer on the metal surface undergo a pretreatment including a phosphating treatment and optionally, a passivating treatment. In the case of substrates with a complicated three-dimensional geometry, particularly substrates with hollow spaces and/or undercuts such as, automotive bodies or automotive body parts, carry-over of pretreatment materials and hence of troublesome ions such as, in particular, phosphate ions, into the CED coating bath may take place, for example, if the rinsing operations forming the conclusion of the pretreatment are not carried out perfectly and with the necessary effectiveness. Troublesome ions of this kind may adversely affect the effectiveness of bismuth in the bismuth-containing CED coating agents known from the prior art. Adverse consequences include, for example, declining catalytic effect with regard to baking-cross-linking of the CED coating layers, and impaired anti-corrosive effect of the baked CED coating layers.
The object of the invention is to provide bismuth-catalyzed CED coating agents with resistance or at least increased resistance to troublesome ions of the kind which occur, in particular, in pretreatment materials used to produce conversion layers on metal substrates, for example, anions such as phosphate, hydrogen phosphate, dihydrogen phosphate, di- and triphosphate or oligomeric phosphates.
It has become apparent that this object may be achieved with waterborne CED coating agents which contain resin solids and optionally pigments, fillers and/or conventional coating additives and contain at least one bismuth complex with ligands selected from the group consisting of hydroxyquinolines, 1,2-dioximes, 1,3-dioximes and N,Nxe2x80x2-diarylthiocarbazides and combinations of said ligands (hereinafter also called xe2x80x9cbismuth complex as definedxe2x80x9d for short, or simply xe2x80x9cbismuth complexxe2x80x9d) in an amount of 0.1 wt. % to 2.5 wt. %, calculated as bismuth and based on the resin solids.